Russia was pouring reinforcements into eastern Ukraine ahead of a new offensive that could begin next week along a front where there have been relentless battles for months, a Ukrainian governor said.
Desperate for Western military aid to arrive, Ukraine anticipates a major offensive could be launched by Russia for "symbolic" reasons around the February 24 anniversary of the invasion, which Moscow persists in calling "a special military operation".
Ukraine is itself planning a spring offensive to recapture lost territory, but awaiting delivery of promised longer-range Western missiles and battle tanks, with some analysts saying the country was months away from being ready.
"We are seeing more and more (Russian) reserves being deployed in our direction, we are seeing more equipment being brought in...," said Serhiy Haidai, Ukraine's governor of the mainly Russian-occupied Luhansk province.
"They bring ammunition that is used differently than before - it is not round-the-clock shelling anymore.
"They are slowly starting to save, getting ready for a full-scale offensive," Haidai told Ukrainian television.
"It will most likely take them 10 days to gather reserves. After February 15 we can expect (this offensive) at any time."
The war is reaching a pivotal point as its first anniversary approaches, with Ukraine no longer making gains as it did in the second half of 2022 and Russia pushing forward with hundreds of thousands of mobilised reserve troops.
President Volodymyr Zelensky said personnel changes on the border and frontline will bolster Ukraine's military efforts amid uncertainty over the future of his defence minister, just as Russia advances in the east for the first time in six months.
In his Monday evening address, Zelensky said he wanted to combine military and managerial experience in local and central government but did not directly address confusion about whether his defence minister, Oleksii Reznikov, would be replaced.
On Sunday, David Arakhamia, head of Zelensky's parliamentary bloc, said Reznikov would be transferred to another ministerial job, but on Monday he wrote that "there will be no personnel changes in the defence sector this week."
Zelensky says he needs to show that Ukraine was a safe steward of billions of dollars of Western military and other aid, and his government is engaged in the biggest political and administrative shake-up since Russia's invasion nearly a year ago.
"In a number of regions, particularly those on the border or on the front line, we will appoint leaders with military experience. Those who can show themselves to be the most effective in defending against existing threats," he said.
The European Union said Zelensky has been invited to take part in a summit of EU leaders, amid reports he could be in Brussels as soon as this week, in what would be only his second known foreign trip since the invasion began.
Zelensky's office did not respond to a request for comment.
Ukraine's Defence Minister Oleksii Reznikov told Ukrainska Pravda on the weekend that intelligence suggested any new Russian offensive would likely come from the east or south.
"Their dream is to expand the land corridor to Crimea in order to continue supplies.
"Therefore, of course, the key risks are: the east, the south, and after that the north," he said. Moscow annexed Crimea in 2014.
Ukrainian defence analyst Oleksandr Kovalenko said a new Russian offensive could come from one of four directions; the eastern Luhansk region, the Donetsk region, the Zaporizhzhia region and the city and port of Mariupol.
"Things are more serious in Donetsk region, particularly around Bakhmut and Avdiivka.
"And the Russians will be boosting their contingents there as well as equipment and paratroops," Kovalenko, from the "Information Resistance group" think tanks, told Ukrainian radio NV.
For months Russia's main target in eastern Ukraine has been Bakhmut, where its state media said the Wagner mercenary group had gained a foothold. Ukraine said on Monday evening that Russian forces had trained tank, mortar and artillery fire there in the past 24 hours.
Kovalenko said Mariupol, captured by Russian forces last May, could be used by the Russians to bring in troops and equipment for a new offensive.
"It could serve as a transport hub for the Russian occupation forces," he said.
Kovalenko said Ukraine's counter-offensive would not happen any time soon and Ukrainian forces would be assuming a defensive position, particularly in Donetsk.
"It may be an active defence, but a defensive position nonetheless. The idea will remain to block any Russian advance," he said.
"Things could change more quickly in other sectors. But this situation could go on for two to two-and-a-half months - that is the time required for providing the tanks for brigades, training and getting everything outfitted."